Multicasting is a technique where information, for example, packets of information, from one location may be sent to other locations, without the need for unneeded duplication. For example, in a system that supports multicasting, one packet may be sent from a source and may be replicated as needed in a network to reach as many users as required.
A multicast may use a multicast group and a multicast group address. A sending device may send information to the group address and only members of the group may receive the multicast data. A group address may take any number of forms. For example, the group address may be a class D IP address.
Various protocols have been developed to send information across networks. One example of a protocol across the Internet is the Real Time Protocol (RTP). The RTP provides a framework for sending real time data over the Internet. The RTP is designed to be independent of the underlying transportation protocol. Details of the RTP may be found in RFC-1889, which is promulgated by the Internet Engineering Task Force (IEFT).
Still another protocol used for signaling is the Session Initiation Protocol (SIP). SIP depends on various messages being sent between different entities in a network. For instance, SIP messages may be requests from servers or clients, or responses to a request.
In one example of a SIP system, a caller device may send an INVITE message to a called party device. The message may be transmitted via a single or multiple proxies to the called party device. The proxy may send a “100 Trying” message to the caller party device to specify that action is being taken on behalf of the caller device, but the called party device has not yet being located.
Once the called party device has been located, the called party device may send a “180 Ringing” message to the proxy and the proxy may relay this message to the caller device. This message indicates that the called party device has been located and notified of the call.
Once the request has been successfully processed, a “200 OK” message may be sent from the called party device to the proxy and from the proxy to the caller party. An audio connection between the caller party device and the called party device may now be established.